A lithographic system is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic systems may be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern may be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic systems include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one exposure, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A substrate may be elastically deformed (bent) by a substrate table that is not flat. Bending of the substrate will induce overlay errors through distortion of the wafer grid. This wafer grid is defined by lines in two orthogonal directions, referred to as the X- and Y-directions. To ensure good overlay, the substrate table should be as flat as possible from chuck to chuck (i.e. from one chuck to another chuck in a single machine, and from one chuck to another chuck in different machines) and it must not change over time. The flatness of the exposure chuck is a resulting characteristic from the flatness of the individual hardware components in the chuck assembly, with the substrate table acting as the interface supporting the wafer. Flatness differences may be introduced through manufacturing tolerances, wear of components or introduction of defects and/or contamination during the lifetime. The impact on overlay of the substrate table not being flat is two-fold. The local wafer grid distortion due to a non-flat surface will cause global alignment errors that are non-representative for the wafer grid. The wafer grid may be deformed locally resulting in a (different) 2-dimensional grid ‘fingerprint’ (i.e. a chuck specific systematic distortion induced by the chuck non-flatness) with random residual field translations and/or local field expansion or rotational errors per field.
Overlay may also be caused by a non-flatness of the mask table.